T cell activation occurs following two molecular interactions commonly referred to as signal one and signal two. Signal one comes from the antigenic peptide-MHC complex interacting with the T cell receptor (TCR) and signal two describes a series of co-stimulatory receptors on antigen presenting cells (APCs) such as the CD28/CTLA-4-CD80/CD86 co-stimulation pathway Riha and Rudd, Self Nonself 2010; 1:231-240; Sansom, Immunology 2000; 101:169-177). Blocking either signal can halt T cell activation.